Proteins with glycosylphosphatidylinositol (GPI) anchors exhibit a range of activities and some of these proteins exist in both a membrane-associated and a soluble form. CD48 is a 47-kd GPI-linked glycoprotein which is expressed on T and B lymphocytes, monocytes, and many lymphoid malignancies. The biological function of CD48 is unknown. We describe the detection of a soluble form of CD48 in plasma and serum. Its level was quantified by an immunoenzymometric assay (IEMA) specific for soluble CD48. While soluble CD48 was detected in the plasma of healthy individuals (median = 29 ng/ml; range, 15-48 ng/ml), elevated levels were detected in some patients with lymphoproliferative disease (median = 41 ng/ml; range, 9-213 ng/ml, arthritis (median = 42 ng/ml; range, 13-67 ng/ml), and acute EBV infection (174 ng/ml). Soluble CD48 was also detectable in tissue culture supernatants from the Raji lymphoid cell line. The mechanism of CD48 release from cells is unclear. The finding of significant levels of soluble CD48 in plasma and the development of a sensitive IEMA for its measurement will facilitate further studies on its normal function and its role in disease.